Research On Protection Method Of UHVDC Transmission Line

With the rapid development and the gradual maturity of UHVDC transmission technology,and it becomes more and more important in the power system,The UHVDC transmission technology has gradually become the main method to solve the problem of high voltage,large capacity,long distance transmission and power grid interconnection.But he safety and reliability of the UHVDC requirements should be greatly improved,so it needs to further analyze the fault characteristics and improve it’s protection program.This paper focuses on the fault characteristics and protection methods of UHVDC transmission lines.First of all,the structure and operation principle of UHVDC transmission system are described in this paper.According to the real parameters of Yunguang UHVDC transmission,a simulation model of the bipolar 12 pulse UHVDC system is bluit by using the simulation software PSCAD/EMTDC,and the typical fault of the transmission line is simulated in this model and obtaining fault signals and fault characteristics.Secondly,a new protective principle is proposed by using two-terminal current correlation,which bases on analyzing faults that occur in internal or external DC transmission line.According to the current changes are in the opposite direction in the internal faults,but the same in the external faults,we can constitute a criterion by comparing current correlation,and to achieve quick and accurate recognition about the faults whether they occur in internal or external DC transmission line.The method does not require accurate time synchronization for protection devices,and it is reliable,practical and meets the requirements for HVDC line protection,therefore.it can be used as the main protection of the line.Then,a double-ended full-line backup protection method is presented based on the characteristic frequency of fault voltage signal.Due to the existence of smoothing reactors and DC filters as well as PLC filter in DC transmission line,they constitute the physical boundaries of DC line.Because it has strong attenuation for high frequency signal,thus the high-frequency information is not the same about faults of inside and outside the region.Using the Chebyshev digital filter to extract 600～700Hz signal of breakdown voltage,through calculating Shannon entropy of one determined frequency of the periodic signal after the fault occurred to accurately identify the fault region.At last,a new back-up protection method for UHVDC transmission lines is presented based on measuring inductance difference under natural frequency.The different features of measuring inductance under internal faults and external faults are analyzed,and the natural frequency of the current signals is estimated by multiple signal classification(MUSIC).Then the amplitudes of voltage and current are calculated by DFT,and the inductance under natural frequency is obtained.The inductance difference is equal to inductance of DC transmission line under external faults,while internal faults would make the inductance difference approximately equal to zero,thus the protection criterion is given based on this phenomenon.The simulation results of PSCAD and MATLAB verify the feasibility and effectiveness of the proposed protection methods for the UHVDC transmission line.